A drive unit for an automatic transmission changes over a hydraulic circuit of the automatic transmission, to implement the gear change. A conventional drive unit of this kind, for example, as shown in FIGS. 6 and 7, has a hydraulic circuit changing device 101 to be manually operated by a shift lever 104 and to change a hydraulic control portion (gear change portions 111 and 112), to which oil pressure is applied. The drive unit 100 further has a position detecting device 102 for continuously detecting a position of a movable member (a spool 105), which is a part of the hydraulic circuit changing device 101, and outputting a detection signal indicating the position of the movable member (which corresponds to an amount of movement of the spool driven by the shift lever 104). An electronic control unit 103 of the drive unit 100 determines the hydraulic gear change portion to which the oil pressure is applied, based on the detection signal from the position detecting device 102, and controls the oil pressure. (In this specification, the drive unit for mechanically changing over the hydraulic circuit in accordance with a shift lever movement by a vehicle-driver is referred to as a “mechanically operated type drive unit”.)
A movable portion of the hydraulic circuit changing device 101 comprises the shift lever 104 operated by the vehicle driver and the spool 105 linearly moved by the shift lever 104. The movable portion of the hydraulic circuit changing device 101 further comprises a control wire 106 connected at its one end to the shift lever 104 and at its other end to a shaft 107, which is rotated in a synchronized manner with the shift lever 104. The movable portion further comprises a valve lever 108, which is swung by the shaft 107. An operational force of the shift lever 104 is mechanically transmitted to the spool 105 through the above movable portion.
The spool 105 is movably held in a body 109 of an automatic transmission device 113, so that the spool 105 is linearly moved back and forth in the body 109 in accordance with the operational force of the shift lever 104. As schematically shown in FIG. 7, the body 109 of the automatic transmission device has an inlet port 109a connected to an oil pressure source 110 and multiple outlet ports 109b and 109c, which are respectively connected to the multiple hydraulic control portions (more specifically, to a gear change portion 111 for a forward drive, and to a gear change portion 112 for a backward drive). The outlet port 109b is referred to as a “D” port connected to the gear change portion 111 for the forward drive, while the outlet port 109c is referred to as a “R” port connected to the gear change portion 112 for the backward drive. When the spool 105 is linearly moved back and forth, the “D” port 109b or “R” port 109c is selectively connected to the oil pressure source 110. FIG. 7 shows a condition, in which the gear change portion 111 is connected to the oil pressure source 110 through the “D” port 109b. 
The position detecting device 102, for example as disclosed in Japanese Patent Publication No.H7-301309, is provided at an outside of the automatic transmission device 113, and comprises a rotating member (not shown) integrally formed with the shaft 107, and a distance sensor (not shown) for outputting an electrical signal corresponding to a distance between the sensor and an outer periphery of the rotating member. The electrical signal corresponding to a rotational angle (the rotational position) of the shaft 107 can be outputted to the electronic control unit (ECU) 103.
As already described above, the electronic control unit 103 determines the hydraulic control portion (the gear change portion 111 or the gear change portion 112), to which the oil pressure is applied, based on the detection signal from the position detecting device 102.
More specifically, the ECU 103 determines that the inlet port 109a is connected to neither “D” port 109b nor “R” port 109c, when the detection signal of the position detecting device 102 is lower than a predetermined level “C”, as shown in FIG. 8B. When the detection signal is lower than the predetermined level “C”, the ECU 103 determines that the shift lever 104 is positioned at a “N” range (a neutral range). When the detection signal is higher than the predetermined level “C”, the ECU 103 determines that the inlet port 109a is connected to the “D” port 109b (which corresponds to a condition that the shift lever 104 is positioned at a “D” range (a drive range)). When the detection signal of the position detecting device 102 is changed from the “N” range to the “D” range, the ECU 103 determines that inlet port 109a is connected to the “D” port 109b. The ECU 103 then controls the oil pressure (also referred to as a control pressure) to be applied to the gear change portion 111 for the forward drive.
The ECU controls the control pressure, in order to alleviate an impact which would be caused by a rapid supply of a working oil to the gear change portion 111. For the purpose, a pressure regulating valve 114 is provided in a hydraulic circuit from the “D” port 109b to the gear change portion 111, as shown in FIG. 7, so that an opening degree of the valve 114 is controlled in accordance with a current supply to a solenoid (not shown) of the valve 114. The ECU 103 changes a command value for controlling the current supply in accordance with a predetermined control program, so that the opening degree of the pressure regulating valve 114 is gradually increased to prevent the oil pressure at an upstream side of the pressure regulating valve 114 from rapidly applied to the gear change portion 111. The oil supply pressure is increased by supplying the working oil from the oil pressure source 110 to the upstream side of the pressure regulating valve 114 through the inlet port 109a and the outlet port 109b (the “D” port 109b).
As seen from FIGS. 8B and 8C, the outlet port (“D” port) 109b is communicated with the inlet port 109a only after the detected position signal from the position detecting device 102 exceeds the predetermined level “C” (i.e. the shift lever 104 is changed from the “N” range to the “D” range. Accordingly, the oil supply pressure to the pressure regulating valve 114 is increased (through the communication between the inlet port 109a and the “D” port 109b), only after the control process for the pressure control valve 114 has started. In a normal operation of the shift lever 104 by the vehicle driver, the shift lever 104 is usually moved quickly. However, if the shift lever 104 is intentionally moved very slowly, the above drive unit for the automatic transmission device has the following problems.
In FIGS. 8A to 8E, a timing point, at which the detected position signal from the position detecting device 102 is changed from the “N” range to the “D” range in the normal shift operation of the shift lever 104, is indicated by “t1a”, and a timing point, at which the “D” port 109b is actually communicated with the inlet port 109a in the normal shift operation of the shift lever 104, is indicated by “t2a”. A timing point, at which the detected position signal from the position detecting device 102 is changed from the “N” range to the “D” range in the intentional slow shift operation of the shift lever 104, is indicated by “t1b”, and a timing point, at which the “D” port 109b is actually communicated with the inlet port 109a in the intentional slow shift operation of the shift lever 104, is indicated by “t2b”. In FIGS. 8A to 8E, dotted lines indicate respective operational conditions with respect to time change in the case of the normal shift operation, whereas solid lines indicate the respective operational conditions with respect to the time change in the case of the intentional slow shift operation.
A moving speed of the movable portion, such as the shift lever 104 and the related elements (the spool 105 and so on), becomes extremely slower in the intentional slow shift operation, than that in the normal shift operation.
In FIG. 8A, “X1” is a threshold level of a stroke amount of the shift lever 104, at which the ECU 103 determines that the shift lever 104 is moved from the “N” range to the “D” range based on the detected position signal from the detecting device 102, namely the ECU 103 determines that the “D” port 109b is communicated with the inlet port 109a. “X2” is, on the other hand, a stroke amount of the shift lever 104, at which the “D” port 109b is actually communicated with the inlet port 109a. As indicated in FIG. 8A, the stroke amounts of “X1” and “X2” are the same in both of the normal shift operation and the intentional slow shift operation.
The drive unit has a delay time in its operation, which is a time period from the timing point (“t1a” or “t1b”) at which the ECU 103 determines that the shift lever 104 is moved from the “N” range to the “D” range to the timing point (“t2a” or “t2b”) at which the “D” port 109b is actually communicated with the inlet port 109a. And the time delay (t2b−t1b) in the case of the intentional slow shift operation becomes much larger than the time delay (t2a−t1a) in the case of the normal shift operation.
As a result, in the case of the intentional slow shift operation, the “D” port 109b is actually communicated with the inlet port 109a, with a considerable time delay, after the ECU 103 has determined that the “D” port 109b is communicated with the inlet port 109a. In such a case, as shown in FIG. 8C, the oil supply pressure is actually increased only at the timing point of “t2b”, in spite that the ECU 103 has already determined at the timing point of “t1b” that the “D” port 109b was communicated with the inlet port 109a. 
As shown in FIG. 8D, the ECU 103 starts its control program for the command signal, as soon as it determines the “D” port 109b is communicated with the inlet port 109a, namely when the detected position signal is changed from the “N” range to the “D” range, irrespectively whether it is in the normal shift operation or the intentional slow shift operation. Accordingly, the opening degree of the pressure regulating valve 114 has already become large, when “D” port 109b is actually communicated with the inlet port 109a. Then, the control pressure to the gear change portion 111 is rapidly increased, as shown in FIG. 8E, to cause an impact in the automatic transmission device.
FIG. 9 also shows another conventional drive unit 100 for the automatic transmission device, in which an electronically operated type drive unit is used. In the electronically operated type drive unit, a movable portion (such as the spool 105) is operated by an electrical actuator 115 (e.g. an electric motor). The electrical actuator 115 is controlled by the ECU 103 in accordance with the shift lever change.
The above explained impact caused in the mechanically operated type drive unit (FIGS. 6 & 7) may also happen in the electronically operated type drive unit (FIG. 9), as in the similar manner.